P1857R2Modules Dependency Discovery

Published Proposal, 2020-01-13

Abstract

The changes to import in [P1703r1] were both too much
and not enough to enable fast dependency scanning via partial preprocessing.
It made import at the start of a line anywhere in the program an import-keyword, breaking things like import->doImport();.
Also, no similar changes were applied to module. This paper
fine-tunes the context-sensitivity of import and module, applying the fixes discussed on the reflector. This
resolves US026, US121, US125, GB126, US127, US128, US136, US137, US138, US139,
US140.

⛔ A token was identified as a modules keyword, but the program is later found
to be ill-formed.

❌ Ill-formed.

☠️ Undefined behaivor

3. Motivation

3.1. Fast Dependency Scanning

Fast dependency scanning relies on partial preprocessing. Several people are
working on fast dependency scanning for modules, and they all share the common
trait of skipping over non-directives. clang-scan-deps is one such tool and is a representative motivating usecase for these changes.

clang-scan-deps works by minimizing source code via partial preprocessing.
Partial preprocessing works by running phases 1-3 of translation. Minimization
proceeds by throwing away every line which is not a preprocessing directive,
then removing all preprocessing directives that can’t impact the set of dependencies for that file.
Up to C++17 this means it only needs to keep #define, #undef, #if*, #else, #elif, #endif, and #include. And only the bodies of #ifs that contain a
preprocessing directive that could impact dependencies. This reduces most files down to their header guards and list of #includes, and is correct for standard C++ except for abuse of __LINE__ such
as #if __LINE__ > 456. The only other things that break it are compiler
extensions which do not follow the parsing rules of # directives such as _Pragma("push_macro(\"X\")"), or .incbin in inline assembly. Additionally,
abuse of the compiler extension __COUNTER__ can break this.

This minimization is context free, and is done once for each file in the
entire build.

Then the minimized source file is fully preprocessed by running phases 1-4, with #includes resolving to their minimized equivalent, macros expanded, and all
preprocessing directives executed.

Minimization followed by using clang’s full preprocessor currently provides
about a 9x speed up (~3 seconds vs ~28 seconds out of a 15m build on a 18 core
iMac Pro) over no minimization at scanning all of llvm and clang (~7k files and ~3.8m LoC). We do not
have access to a large C++20 modules only (no header units) codebase to do tests
on, but that case still requires scanning the entire file due to the potential
for #include based x-macros which will not be going away with modules, as they are intentionally non-modular
includes. There will still be a difference
in that we no longer have normal includes, and thus don’t need to preprocess a
header for each TU it’s used in.

Of the 3s only 33ms are spent minimizing, while the majority of the time is spent in the full lexer, preprocessor, and header
search. We believe we can reduce this overhead.
This overhead is important as it’s on the critical path. Any
speedup here turns into a direct reduction in build latency, which is important
for extremely parallel builds.

C++20 currently breaks this approach. [P1703r1] resolved the issue for import, but dependency scanning also needs to find module declarations, and
the current rules are not enough.

3.2. import is Too Relaxed

[P1703r1] went too far in fixing the dependency scanning issue with import.
Now any line starting with import is treated as an import directive, even if
it obviously couldn’t be. This breaks a decent amount of real code with
function arguments or local variables named import, as any use of them without
a prefix is treated as an import directive, and prepending :: doesn’t work,
you must wrap some part of the expression in ().

4. Discussion

4.1. Implementation

There were some concerns expressed during the discussion of [P1703r1] that
there may be performance or complexity issues. I have implemented this fix
in Clang. It was rather simple to implement, with the most complex part being
doing token lookahead while lexing (which isn’t that complicated). Measurment
did not show any performance impact, which I expect to be due to the codepath
only occuring with the import and module tokens.

4.2. One Line Restriction

To be fully resiliant, a modules directive must be entirely on a single
logical line. You could have a rule that the ; must not come from a macro and
say that the directive extends to the next ;, but that is incorrect:

#define f(x) "blah" #x "blah"importf(;);

4.3. Extra Dependencies

A preprocessing directive consists of a sequence of preprocessing tokens that
satisfies the following constraints: The first token in the sequence, referred
to as a directive-introducing token, is a # preprocessing token, an import
preprocessing token, or an export preprocessing token immediately followed by
an import preprocessing token, that (at the start of translation phase 4)
either begins with the first character in the source file (optionally after
white space containing no new-line characters) or follows white space
containing at least one new-line character.

If there are sequences of preprocessing tokens within the list of [macro]
arguments that would otherwise act as preprocessing directives, the behavior
is undefined.

A minimizer will transform this into just import<a>; which when preprocessed
will form a valid module import. There are two outcomes of this:

The dependency scanner emits an error because <a> is not a header unit or
it is able to detect the usage in a macro.

The dependency scanner succeeds and then the compiler sees the code and
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This is not the best outcome, as an implementation is not guaranteed to emit a
diagnosic, from my tests of clang, msvc, gcc, and icc, only clang rejectes #include in this case, and no compiler rejects an import directive (I plan
to fix this in clang as part of implementing P1703 and this paper). This should
be fixed by changing [cpp.replace]/11 to be ill-formed instead of UB, but that deserves a separate paper. With that
change though the possible outcomes would be:

The dependency scanner emits an error because <a> is not a header unit or
it is able to detect the usage in a macro.

The dependency scanner succeeds and then the compiler sees the code and
rejects it because directives cannot appear as a macro argument.

Both of these outcomes are fine as their result is the same. If a partial
preprocessing would ever get the dependencies wrong, the compiler will reject
the program as ill-formed.

4.4. Ship Vehicle

This needs to ship with modules as this is a backwards incompatible change.
Thus it needs to be in C++20.

5. Wording

5.1. [lex.pptoken]

4 The import-keyword is produced by processing an import directive
([cpp.import])
, and the module-keyword is produced by preprocessing a
module directive ([cpp.module]), and the export-keyword is produced by
preprocessing either of the previous two directives.and has no[Note: None have anyassociated grammar productionsobservable spelling
.
—end note]

5.6. [module.global]

5.7. [cpp.pre]

1 A preprocessing directive consists of a sequence of preprocessing tokens that satisfies the following constraints:
At the start of translation phase 4, theThe
first token in the sequence, referred to as a directive-introducing token,
begins with the first character in the source file (optionally after white space containing no new-line characters) or follows white space containing at least one new-line character, and
is

a # preprocessing token,
or

an import preprocessing token
, or an export preprocessing token immediately followed by an import preprocessing token,immediately followed on the same logical line by by a header-name, <, identifier, or : preprocessing token, or

a module preprocessing token immediately followed on the same logical line by an identifier, :, or ; preprocessing token, or

an export preprocessing token immediately followed on the same logical line by one of the two preceding forms.

that (at the start of translation phase 4) either begins with the first character in the source file (optionally after white space containing no new-line characters) or follows white space containing at least one new-line character.
The last token in the sequence is the first
token in the sequence that is immediately followed by whitespace containing a
new-line character that follows the first token in the sequence. A new-line character ends the preprocessing directive even if it occurs within what would otherwise be an invocation of a function-like macro.
[Example:

# // preprocessing directivemodule;// preprocessing directiveexportmoduleleftpad;// preprocessing directiveimport<string>;// preprocessing directiveexportimport"squee";// preprocessing directiveimportrightpad;// preprocessing directiveimport:part;// preprocessing directivemodule// not a preprocessing directive;// not a preprocessing directiveexport// not a preprocessing directiveimport// not a preprocessing directivefoo;// not a preprocessing directiveexport// not a preprocessing directiveimportfoo;// preprocessing directive (ill-formed at phase 7)import::// not a preprocessing directiveimport->// not a preprocessing directive

2 A
text line shall not begin with a # preprocessing token sequence of preprocessing tokens is only a text-line if it does not begin with a directive-introducing token
. A conditionally-supported-directive shall not begin with any of the
directive names appearing
after a #
in the syntax. A conditionally-supported-directive is conditionally-supported with implementation-defined semantics.

3At the start of phase 4 of translation the group of a pp-global-module-fragment shall neither contain a control-line not starting with a # preprocessing token nor a text-line.

5.8. [cpp.module]

Add a new section before [cpp.import]: Module directive [cpp.module]

pp-module:exportoptmodulepp-tokensopt;new-line

1 A pp-module shall neither appear in a context where module is an
identifier defined as an object-like macro nor where export is an identifier
defined as an object-like macro if the first token of the pp-module is export.

2 Any preprocessing tokens after the module preprocessing
token in the modulecontrol-line are
processed just as in normal text. [Note: Each identifier currently defined as a
macro name is replaced by its replacement list of preprocessing tokens. —end note]

3 The module and export (if it exists) preprocessing tokens are replaced by the module-keyword and export-keyword preprocessing tokens respectively. [Note: This makes the line no longer a
directive so it is not removed at the end of phase 4. —end note]

5.9. [cpp.import]

Insert a new paragraph immediately after the grammar.

A pp-import shall neither appear in a context where import is an
identifier defined as an object-like macro nor where export is an identifier
defined as an object-like macro if the first token of the pp-import is export.

Insert a new paragraph after paragraph 1

If an import-directive is produced by source file inclusion (including by the rewrite produced when a #include directive names an importable header) while processing the group of a module-file, the program is ill-formed.

Update paragraph 2

In all three forms of pp-import, the importand export (if it exists)tokenpreprocessing tokensisare
replaced by the import-keywordand export-keywordtokenpreprocessing tokens respectively
.
[Note: This makes the line no longer a directive so it is not removed at the end of phase 4. —end note]
Additionally, in the second form of pp-import, a header-name token is formed as if the header-name-tokens were the pp-tokens of a #include directive. The header-name-tokens are replaced by the header-name token. [Note: This ensures that imports are treated consistently by the preprocessor and later phases of translation. —end note]

1
If the first two preprocessing tokens at the start of phase 4 of translation are module ;, the result of preprocessing shall begin with a pp-global-module-fragment for which all preprocessing-tokens in the pp-balanced-token-seq were produced directly or indirectly by source file inclusion ([cpp.include]), and for which the second module preprocessing-token was not produced by source file inclusion or macro replacement ([cpp.replace]). Otherwise, the first two preprocessing tokens at the end of phase 4 of translation shall not be module ;.

5.11. [diff.cpp17.basic]

The editor should consider a different section of annex C to move this to.

Affected subclauses: [basic.link], [module.unit], and [module.import] Change: New identifiers with special meaning. Rationale: Required for new features. Effect on original feature:
Top-level declarationsLogical lines
beginning with module or import may be
either ill-formed or
interpreted differently in this International Standard. [ Example: